LONDON — It feels as if the world is facing an unprecedented number of global security crises, NATO's former Supreme Allied Commander said ahead of a key gathering for the 28-nation military alliance that will see heads of state, including President Obama, travel to Wales this week.

Adm. James Stavridis, NATO's highest-ranking military officer from 2009-13, and now dean of Tufts University's Fletcher School of Law and Diplomacy, said the Sept. 4-5 summit — taking place just outside the Welsh capital of Cardiff, in the town of Newport — is "clearly the most important one since the fall of the Berlin Wall because of the clear level of multi-crises" unfolding around the world all at once.

Among the major areas of concern Stavridis identified Islamic State activities in Syria and Iraq, and along the Turkish border; the continuing fallout from the Arab Spring (in Libya and Egypt); the winding down of NATO's mission in Afghanistan and the challenge of setting up a longer-term mission there; escalating concerns over cyberthreats; territorial disputes in the South China Sea; what he called the "classic black swan" that is the Ebola virus; ever-more-forceful environmental catastrophes; and "many others."

However, Stavridis said that the issue that will dominate summit discussions by Obama, German President Angela Merkel, French President Francois Hollande and some 60 other leaders and senior diplomats will be what to do about "Russian adventurism" in eastern and southern Ukraine.

The meeting is NATO's first major jamboree since 2012, in Chicago, where enlargement of the alliance and the deepening of partnerships were salient parts of deliberations. But just a few years later the institution that was set up in 1949 to push back against postwar Soviet expansionism, prevent European nationalist militarism and to encourage political integration is being challenged anew by Russia's alleged backing of separatist fighters over the border in Ukraine.

Ukraine's government in Kiev alleges with NATO support — and Moscow denies — that Russian troops are being actively deployed in the east and south of the country, a standoff that is threatening direct military confrontation between the two neighboring states, and that has the potential to pull in NATO's biggest military powers, such as the USA and Britain.

"If NATO is not seen to react meaningfully (to Russian action in Ukraine) then it doesn't much matter what it says about the Middle East or events in the Indian Ocean or anything else. In one sense this is a summit with a long agenda, but actually, there's only really one item on it," said Michael Clarke, the director general of the Royal United Services Institute, a London-based think tank that specializes in defense and security research. "NATO has to come up with a set of policies that can deal with Russian subversion."

But Xenia Wickett, who previously served in the State Department in South East Asia and now runs the USA program at Chatham House, an international affairs institute in London, said in a briefing for journalists on Thursday that the summit in Newport should be seen as a "way station" rather than an "endpoint" in terms of dealing with NATO's current and emerging security challenges.

"There's a risk that the summit becomes all about putting out fires rather than a more strategic one," she said. "The summit needs to pay attention to current events, but it also needs to look much further ahead in terms of where NATO (as an organization) is going."

Under NATO's Article 5 an attack against one NATO member is an attack against all members. Ukraine is not a full member of NATO but a "partner," which means it cooperates on a range of issues with the alliance, including receiving some military assistance.

However, Kiev has indicated that it will seek full NATO membership, and Article 5 does in theory apply to the Baltic states of Estonia, Latvia and Lithuania, which sit on Russia's doorstep.

President Obama, who will travel to Estonia on Tuesday to meet with all three Baltic leaders, is expected to send a strong signal to Russia about its alleged aggression in Ukraine and to reconfirm the USA's commitment to NATO's Article 5.

There are other topics of major significance for NATO that will be addressed in Wales. Not least, NATO leaders are likely to say something about what kind of defense posture the alliance will take in Afghanistan next year given that its responsibility for domestic security will pass almost entirely to Afghans at the end of this year.

A statement about NATO spending is also expected. Only a few NATO members currently spend the target of 2% of GDP on defense. The USA spends closer to 4%, according to World Bank data. The crisis in Ukraine means some counties may be asked to commit to more spending.

Still, while NATO today appears to be facing a multiplication of conflicts led by state and non-states, the nature of the threats is not necessarily as grave as in previous years, said Robin Niblett, the director of Chatham House, during the briefing in London.

"The existential threat that NATO members faced through the Cold War we do not face today, even from terrorist attacks — as dreadful and bloody and damaging as they can be," he said. (On Friday, Britain raised its level of threat from international terrorism from "substantial" to "severe," the second-highest level, suggesting an attack is highly likely.)

"The idea of nuclear Holocaust, which we were living with through that period, we must remember that we are not living in that world now."